CA2304183C

CA2304183C - Universal flow channel

Info

Publication number: CA2304183C
Application number: CA002304183A
Authority: CA
Inventors: Ching-Chih Lee; Ernest Wilford Looman, Jr.; Gary Robert Burg; Bruce Joseph Turner; Malcolm George Marshall
Original assignee: Goodyear Tire and Rubber Co
Current assignee: Goodyear Tire and Rubber Co
Priority date: 1997-10-31
Filing date: 1997-10-31
Publication date: 2005-08-09
Anticipated expiration: 2017-10-31
Also published as: EP1034070A1; WO1999022927A1; DE69734022D1; DE69734022T2; JP2001521838A; CA2304183A1; AU5103598A; EP1034070B1; JP4121700B2

Abstract

Apparatus for directing the flow of material from a supply source to one or more shaping dies has a flow channel therein. A flow directing insert (34A) located in the downstream region of the flow channel spreads the material toward the outer walls and improves uniformity of flow in an exit zone (40). In the downstream region (11), the channel height is gradually decreased and levels off near the exit. The flow channel may be split into two or more flow branches (15A, 15B), each flow branch having a flow directing insert (34A) therein.

Description

KW . 1 u:~ : ~rA :N~. ~:.~rtL.v a 1 : t~- :~-as : cu : uu : -. r~ ~ ua v;sa~~~rc~,~ : rr ~t ~Sop-24-99 Ol:filP P.04 UNIVERSAL FLOW CHANNFI.
Technical Field This invention pertains to the art of methods and apparatuses for directing the Flow of ~lymeric material from a supply source to one or more shaping die.S.
_I~ck round Art In the art it is known to provide means for directing the flaw of various types of polymeric material from a supply source such as an extruder to one or more shaping dies. There are ~~cr'ail factors which affect how the material flows within the flow channel. Considerations of wall friction, stretching and compression of the material, pressure loss and clamping force arc i~dctored in flow channel construction.
For It) example, wall friction tends to slow the flow of material near the walls so that the flow of cnateiia! near the center is ~dstxr, producing undesirable results.
European Patent Document Nu. 0 226 434 discloses a polymer sheet produced by advancing a workpiece of polymeric material in the longitudinal direction between two pairs of opposed wc~rlcing surfaces of a die, so that the material of the werkpiece 15 flows simultaneously in the longitudinal direction (L) anti the lateral direction (Y) while decreasing in thickness in the (X) direction. A first pair of working surfaces of the die irvclude upp~scd protrusions that squeeze the workpiecc awre at the center than at the edges. In this disclosure, the outlet ends of the protrusions extend the entire width of the channel between the second pair of working surfaces and the workpiccc is pressed 20 between tow protrusions, Japanese Patent Document No. 0327U5~9 discloses a pair of truncated quadrangular pyramid-like projections pro~~det1 in a converging die heat!. The width of each projection increases toward the efflux port and the top Faces of the projections are nearly parallel to each other. The efflux port end of cacti of the projections is sloped 2~ away from the top face which provides a slight inc;rrage in depth of the flow channel.
Moving the tnaterial through turns or overlaround objects may cause unpredictable viscn-elastic effetas. The problem of non-uniform mtrterial flow and undesirahlc visco-elastic effects are addressexl in the present invention.
The present invention is directed to the downstream region of a flow channel in 30 an apparatus for directing the flow of polymeric material from a supply source to one ur more shaping dies. The inventive c;on.~strnction of the downstream region provides fvr increased uniformity of flow across the entire width of the exit orifice. The present invention is further directed to balancing the flow of polymeric material between two or AMENDED SHEET

more flow passages.
Disclosure of the Invention In accordance with the practice of the present invention, there is provided a new and improved method and apparatus for directing the flow of material from a supply source to one or more shaping dies.
In accordance with another aspect of the present invention, there is provided an apparatus for directing the flow of polymeric material from a supply source to one or more shaping dies, the apparatus having a flow channel therein, the flow channel having an inlet communicating with a supply orifice for receiving the material, the flow channel being defined by opposite side walls, an upper wall, and a lower wall, the flow channel including a downstream region having a first zone in flow communication with the inlet, and a second zone in flow communication with the first zone, the first and second zones being associated with a common centerline, the side walls of the second zone diverging in opposite lateral directions generally symmetrically about the centerline and associated with an angle, a, the upper wall being in a common plane throughout the first and second zones, and the apparatus further having a flow directing insert located within the second zone positioned adjacent to the lower wall and spaced from the opposite side walls, the flow directing insert having sloped side surfaces intersecting a generally planar top surface, the top surface being spaced from the upper wall and generally parallel thereto.
According to another aspect of the invention, the flow channel includes a third zone in flow communication with the second zone for directing the flow of the material into the one or more shaping dies, the lower wall of the third zone being generally flush with the top surface of the flow directing insert.
According to another aspect of the invention, the third zone includes an exit orifice, the exit orifice being associated with a flow channel width, wherein the exit orifice width is greater than an associated maximum lateral width of the flow directing insert.
According to yet another aspect of the invention, the upper wall and the lower wall of the flow channel in the second zone are disposed at an angle (3 to one another to affect a gradual decrease in a channel height in the second zone.
According to yet another aspect of the invention, the downstream region of the flow channel includes a pair of diverging flow branches in flow communication with the inlet, each of the flow branches including a flow directing insert located within each of the second zones.
In accordance with another aspect of the present invention, there is provided apparatus for directing the flow of polymeric material from a supply source to one or more shaping dies, the apparatus having a flow channel therein, the flow channel having an inlet communicating with a supply orifice for receiving the material, the flow channel including a downstream region having a pair of diverging flow branches in flow communication with the inlet, each of the flow branches being defined by opposite side walls, an upper wall, and a lower wall and including a first zone in flow communication with the inlet, and a second zone in flow communication with the first zone, the first and second zones being associated with a common centerline, the side walls of the second zone diverging in opposite lateral directions generally symmetrically about the centerline and associated with an angle, a, the apparatus including means for restricting the flow of the polymeric material along the centerlines while urging the polymeric material toward the respective opposite side walls, each of the upper walls being in a common plane throughout the first and second zones, the apparatus further having a flow directing insert located within each second zone positioned adjacent to the lower wall and spaced from the opposite side walls, the flow directing insert having sloped side surfaces intersecting a generally planar top surface, the top surface being spaced from the upper wall and generally parallel thereto.
In accordance with another aspect of the present invention, there is provided a flow directing insert for use in an apparatus for directing the flow of polymeric material from a supply source to one or more shaping dies, the apparatus having a flow channel therein, the flow channel having an inlet communicating with a supply orifice for receiving the material, the flow channel being defined by opposite side walls, an upper wall, and a lower wall, the flow channel including a downstream region having a first zone in flow communication with the inlet, and a second zone in flow communication with the first zone, the first and second zones being associated with a common centerline, the side walls of the second zone diverging in opposite lateral directions generally symmetrically about the centerline and associated with au -3a-angle, a, the upper wall being in a common plane throughout the first and second zones, the flow directing insert for location within the second zone for positioning adjacent to the lower wall and spaced from the opposite side walls, the flow directing insert having a triangular bottom surface, a triangular top surface non-parallel to the bottom surface, and sloping side surfaces extending between the bottom surface and the top surface, the side surfaces intersecting to form a forward edge.
One advantage of the present invention is the reduction of visco-elastic effects in the extruded material.
Another advantage of the present invention is the improved uniformity of flow of polymeric material across the entire width of an exit orifice.
Another advantage of the present invention is that the flow of material between two separate flow branches of a flow channel is balanced.
Another advantage of the invention is the reduction of the projected area of the region filled with the polymeric material which decreases the required clamping force for a predetermined flow rate.
Still other benefits and advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following specification.
Brief Description of the Drawings The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein:
Figure 1 is a side sectional view of an apparatus for directing the flow of polymeric materials from an extruder to one or more shaping dies.
Figure 2 is a top sectional view of a downstream region of a flow channel.
Figure 3 is a side sectional view of the downstream region of a flow channel taken along the line 3-3 of Figure 2.
Figures 3A and 3B are enlarged views of a section of Figure 3.
Figures 4-7 are cross-sectional views of the downstream region of the flow channel.
Figure 8 is a schematic representation of velocity vectors associated with the flow of polymeric material in the flow channel of the present invention.

-3b-Figures 9-12 are top views of various embodiments of a flow directing insert according to the invention.
Figure 13 is an exploded view of a shaping die assembly for shaping extruded Ill: v . 1 UV : HY.1 .lll,tw.~ HH:\ Ii i : y.l. _ H_~:J : '.u - a t : -. ,-.y;~ ua G:aa:rl *c>-~ . R
-SQp-24-99 01:52P P.07 .4_ iridtCTial.
Figure 14 is a top sectional view of another embodiment of the downstream region of a flow channel according to the invention.
Detailed Descri lion of the Invention g Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the invention nnly sad not for purposes of limiting the same, Figure 1 shows an apparatus H having at least one Dow channel 10 therein for directing the flow of polymeric material. The present invention is directed to the design characteristics of n flow chanctel 10 which provide more unifucm flow to the shaping die, particularly when the material is polytnerie in nature. In particular, the present invention is directed to the downstream region 11 of flow channel 10 just prior to the material flowing into a die assembly 46. Flow channel 10 uicludes an inlet passage 12 which communicates with a sully source as is known in the art and is not shown here. It is contemplated that the invention prnvides the best results when the 1.5 polymeric material has a symmetric velocity distribution with respect to the centerline of the flow crass-section immediately prior to flowing into the downstream region 11.
With respect to Figure 2, a sec;tioral view of a downstream region 11 of one embodiment of a flaw channel is shown. In this embodinxnt, the flow of polymeric material from the supply source, not shown, is split into first and second streams, indicted by arrows A, B before entering downstream region 11. The downstream rcgic~n 11 is generally synunetric about centerline 1~4 and includes first and second flow branches 15A, 15B. Each of the branches 15A, 15B also have respective centerlines 18A, 18B which provide general lateral symmetry within the branches. Located within each of the flow branches 15A, 158 is a flow directing insert 34A.
L5 With particular attention to the right branch 15 A, the features of the downstreans region 11 of flow channel 10 will lx more fully described. For ease of illustration, the downstream region I1 of flow channel 10 will be described in terms of "upper walls", "lower walls". "lateral direction", ate. in aca;ordancc with the:wcompanying drawings.
However, it should be readily understood by chase skilled in the relevant art that the ~p apparatus 8 may have other orientations during use.
The right branch 15A is a continuous pacgage from entry zone 26 to exit zone 40. The entry zoae 26, rrceivec the preferably sytnrnetric flow of polymeric material.
Generally, the flow in the entry zone 26 is in a direction parallel to the main centerline 14. In a second zone, also called the spread out and balancing zone 28, the width of the i -~ .. ~-~~ - ~ ...- a.. ..a -,w.~a aw~ n a SQp- 24 -99 V~O 1 ~: 62 P p - p8 first branch 15A increases in a uniform manner symmetric about right branch centerline 18A. In the spread out and balancing zone 28, the side walls 38, 39 diverge W
form a triangular region associated with an included angle, tx. The included angle, cx, may he between 4Il° and 120°. In the preferred embodiment, angle a is i~etwccn fiU° and 900~
The flow-directing insert 34A is positioned within the spread out and balancing zone 2f3.
As shown in Figure 2, the insert 34A is spaced from each of the side wails 38, 39 w that tlu flow of material is not interrupted near side walls 38, 39. The shape of the insert 34A urges the polymeric material toward the side walls 38, 39 of the right branch 15A by restricting the flow along the centerline 18A. The flow-directing insert 34A
lU further acts tv spread out and balance the flew, ant c:vmpress the polymeric material.
With reference to Figure 3, in the downstream region 11 of flow channel 10, the dashed lines serve to illustrate the location of entry zone 26, spread out and balancing zone 28, and exit zone 40. As is shown, the flew directing insert 34A includes a generally planar top surface 41 and a hcntom surface 43 which is positioned adjacent tct the lower wall 48.
Another feature of tha invention illustrated in Figure 3 is the preferred arrangement of upper wall 50 with respect to lower wall 48. Upper wall 5U and lower wall 48 are preferably not parallel, but are disposed at an angle ~i to one another for gradually decreasing the channel height along the length of the downstream region. The preferred range for ~ is between 5° and 30°.
YCt another feature of the invention illactrated in Figure 3 is the more abrupt decrease in the flow channel bright at the ceater of the flow branch 15A due to the presence of flow directing insert 34A. ThrouLhout the downstream region 11 of the flew channel 10, the channel height decreases without increasing, even in the region encompassing the flaw directing insert 34A. 1t has been found that using a flow-directing insert which first compresses the polymeric rnaterial and then allows expansion in the Itow creates undesirable viscv-elasrtic effects in the polymeric material.
TheIefUIC, the preferred flow-directing insert 34A caucac the channel bright in the spread out and balancing zone 28 to decrease monotonieally and does not deepen again 34 as the polymeric material flows over the insert 34A. The top surface 41 of the flow directing insert 34A is generally planar. In the exit zone 40, the bottom wall 48 of the flow branch 15A is also preferably planar. 1n a preferred embodiment, the bottom wall 48 is flush with the plane of the top surfmc:e 41. In a practical application, thCTe may be a slight step-down at the end of the insert 34A as shown in Figure 38. The preferred .. ~ a .~-a.i ~ sv ~ v.: ~ -» t~J tf;1 ~:3:1a~-l~tW : # J
5Qp- 24-99y~01~:~ 52 P p . pg flow directing insert 34A includes sloped side walls 42. In the preferred embodiment, only a single insert 34A is utilized. The upper wall 50 of the flow channel remains in a single plane, parallel co top surface 41, throughout the length of the flow channel 15A.
This arrangement is simpler in design than some prior art apparatus which provide a pair ~f protruding surfaces.
The length, L, of the exit zone 40 is herein defined as ihc distance from the end of the flow-directing insert 34A to an exit orittce 44. The length of the exit zone 40 effcc;ts bath the pressure loss dnd the vises-elastic effects. The longer the exit zone 40, the greater the pressure loss and the lower the visc;o-elastic effects and vice-versa.
Therefore, in the preferred embodiment, the length of the exit. zone 40 is optimized to provide aecepteble ranges of pressure loss and visco-elastic effects.
With particular reference to higures a-7, the variances in the cross-sectional shape of a preferred embodiment of the right branch 15A of the downstream region 11 are Shawn. Each of the sectional views are taken perpendicular to the ee»terline 18A of IS the right branch 15A. For the sake of simplicity, right branch ISA will be discussed in detail below. Iiowever, left branch 1511, is a mirror image of right branch 15A with centerline 14 being the axis of symmetry.
Figure 4 is a sectional view of the entry zone 26 showing upper wall S0, lower wall 48, ana side walls 38, 39. As is shown, in the preferred embodiment, the junctions 2U of Power wall 37 with side walls 38. 39 are contoured.
>~igure S is directed to the initial portion of the spread out and balancing zone 28 and S11QW5 how the flow-directing insert 34A effects the cross-sectional shape of right branch 15A. 1'he flow-directing insert 34A is generally symmetrically positioned within right branch 15A. In other wurdg, a centerline of the flow-directing insert is aligned 25 with the centerline of the right branch 15A in the 5-pread gut and balancing zone 28. In the spread out and balancing zone 28, the associated width of right branch 15A
gradually increases along a length thereof, and the channel height gradually decreases.
Figure b shows a sectional view further duwnstrcatn than that shcawn in Figure 5, but before the exit zoae 40.
30 Figure 7 is a sectional view of the exit Dana 40. In a preferred embodiment, the channel height remains constant in the exit zone 4p. It is, however, within the scope of the present invention to derreace the channel bright along its length in exit zone 40.
Referring again to Figure Z, in one embodiment of the invention, the exit zone 40 is associated with a lateral width which is greater than the maxin~utn width oP first ., ".~."..:~..., ". _Y- L-:J:1 . ._u.u~y ; y +.~.~ a~ y~~J:)4-4t3s:atu Sap-24-99 01:62P P.10 branch 15A. It is within the scope of the present invention for exit zone 40 tn extend laterally so that it communicates with both right flow branch 15A and left Ilow branch 15B.
With respect tn Figure 8, the features of the downstream region 11 cause the exiting vectors 60 associated with the velocity of the polymeric material to be substantially equal and parallel to the centerline 18A across the entire width ~f the exit 2onc 4U. In prior art extruder Ilow channels, the flow tends to be faster in the center, which causes poor gauge control. Such undesirable effects are greatly reduced by the flow channel construction of the present invention baause of the balanced flow and substantially uniform velocity across the exit none 40.
The design of the downstream region 11 of flow channel 10 of the present invention can be used for single and multiple cavity extrusions. When the polymeric material passes from the exit zone 40 through more than one die cavity, improvement is seen in a balancing of the flow between cavities as well as in uniformity within a given cavity. This versatility for single and multiple cavity extrusions serves to reduce ur eliminate flow channel change-ovens, and thus increa.~c productivity.
The required clamping force for a speciftcJ flow rate of polymeric material is reduced in the flow channel IU of the present invention because the island lb dividing the two streams A and B of Figure 2 greatly reduces the projected arcs associated with 2U required clamping force. Prior art extruders are equipped to provide higher clanipins force for the same flow rate than needed with the flow channel 10. Therefore, use of the inventive flow channel 10 with prior art extruders may allow for increased llow rate ur throughput when utilizing similar clamping forces.
Referring now to Figures 9-12 different embodiments of a flow~iirecting insert 34B-34E are shown. The design of inserts 34$-34F peeks to achieve uniform gap-wise average velocity across the exit opening, while minimizing pressure loss.
Figure 13 is an exploded vices of a shaping die assembly 46 able to recxive material that flows from the tlow channel 10.
In another embodiment of the invention, shown in Figure 14, the flow of polymeric meterial enters the downstream region 11'. In this embodiment, the features of the flow channel 10' are similar to these described above with respeca to the entry zone 26' and the spread out and balancing rune 28' . In the exit zone 4Q' , the lateral width is slightly greater than the maximum width of the flow directing insert 34A'. In this embodiment, the exit zone 40' dons oat ec~rnmunicate with a second flow branch as ,,-,,~ . .;;,,. . ,» . - t~~ ~s~ vsaa~~tt~.p : # 1 1 Sap- 24-99 ~~O 1~:~~62P P . 11 _$_ shown in the embodiment of Figure Z. h is within dtc scope of the pr~cent invention to grovide an apparatus 8 having croe or more of the flow channels lU' shown in Figure 14, rack having discreet exit zones 40' .
It is further within the swipe of die gresent invention to prcwide an apparatus with more than the two flow branches 15A, 15B as illustrated in Figure 2, wherein the CXit ZOI1C 4U 1S CUntlnunuR. It is further within the scope of the present invention to provide any combination of flow channclslflow branches having continuous exit zones 40 «r discreet exit zones 4(l' .
The preferred etnbodintents of the invention have been described, hereinabovr.
14 It will be apparent w those skilled in the art that the above methods may inwtporate changes and modifications without departing from the general st;ope of this invention. It is intended to include all such modifications atul alterations in so the as they corns within the scope of the apgended claims or the equivalents thereof.
Having thus drsc;ribed the invention, it is now claimed:
AMENDED SHEET

Claims

WHAT IS CLAIMED IS:

1. Apparatus for directing the flow of polymeric material from a supply source to one or more shaping dies, the apparatus having a flow channel therein, the flow channel having an inlet communicating with a supply orifice for receiving the material, the flow channel being defined by opposite side walls, an upper wall, and a lower wall, the flow channel including a downstream region having a first zone in flow communication with the inlet, and a second zone in flow communication with the first zone, the first and second zones being associated with a common centerline, the side walls of the second zone diverging in opposite lateral directions generally symmetrically about the centerline and associated with an angle, .alpha., the upper wall being in a common plane throughout the first and second zones, and the apparatus further having a flow directing insert located within the second zone positioned adjacent to the lower wall and spaced from the opposite side walls, the flow directing insert having sloped side surfaces intersecting a generally planar top surface, the top surface being spaced from the upper wall and generally parallel thereto.

2. The apparatus of claim 1 further having:
a third zone in flow communication with the second zone for directing the flow of the material into the one or more shaping dies, the lower wall of the third zone being generally flush with the planar top surface of the flow directing insert.

3. The apparatus of claim 1 further having:
a third zone in flow communication with the second zone for directing the flow of the material into the one or more shaping dies, the lower wall of the third zone being associated with a plane spaced from the plane of the top surface of the flow directing insert.

4. The apparatus of claim 2 wherein:
the flow directing insert has an associated maximum lateral width; and, the third zone includes an exit orifice, the exit orifice being associated with an exit orifice width, wherein the exit orifice width is greater than the associated maximum lateral width of the flow directing insert.

5. The apparatus of claim 3 wherein:
the flow directing insert has an associated maximum lateral width; and, the third zone includes an exit orifice, the exit orifice being associated with an exit orifice width, wherein the exit orifice width is greater than the associated maximum lateral width of the flow directing insert.

6. The apparatus of claim 1 wherein .alpha. is between 40° and 120°.

7. The apparatus of claim 1 wherein the upper wall and the lower wall of the flow channel in the second zone are disposed at an angle .beta. to one another to affect a gradual decrease in a channel height in the second zone.

8. The apparatus of claim 7 wherein angle .beta. is between 5° and 30°.

9. Apparatus for directing the flow of polymeric material from a supply source to one or more shaping dies, the apparatus having a flow channel therein, the flow channel having an inlet communicating with a supply orifice for receiving the material, the flow channel including a downstream region having a pair of diverging flow branches in flow communication with the inlet, each of the flow branches being defined by opposite side walls, an upper wall, and a lower wall and including a first zone in flow communication with the inlet, and a second zone in flow communication with the first zone, the first and second zones being associated with a common centerline, the side walls of the second zone diverging in opposite lateral directions generally symmetrically about the centerline and associated with an angle, .alpha., the apparatus including means for restricting the flow of the polymeric material along the centerlines while urging the polymeric material toward the respective opposite side walls, each of the upper walls being in a common plane throughout the first and second zones, the apparatus further having a flow directing insert located within each second zone positioned adjacent to the lower wall and spaced from the opposite side walls, the flow directing insert having sloped side surfaces intersecting a generally planar top surface, the top surface being spaced from the upper wall and generally parallel thereto.

10. The apparatus of claim 9 wherein:
each of the flow branches includes a third zone in flow communication with the second zone for directing the flow of the material into the one or more shaping dies, the lower wall of the third zone being generally flush with the planar top surface of the flow directing insert.

11. The apparatus of claim 9 wherein:
each of the flow branches includes a third zone in flow communication with the second zone for directing the flow of the material into the one or more shaping dies, the lower wall of the third zone being associated with a plane spaced from the plane of the top surface of the flow directing insert.

12. The apparatus of claim 10 wherein:
the flow directing insert has an associated maximum lateral width; and, each of the third zones includes an exit orifice, the exit orifice being associated with an exit orifice width, wherein the exit orifice width is greater than the associated maximum lateral width of the flow directing insert.

13. The apparatus of claim 11 wherein:
the flow directing insert has an associated maximum lateral width; and, each of the third zones includes an exit orifice, the exit orifice being associated with an exit orifice width, wherein the exit orifice width is greater than the associated maximum lateral width of the flow directing insert.

14. The apparatus of claim 10 wherein:
each of the flow branches includes a third zone; and, an exit orifice common to each of the third zones.

15. The apparatus of claim 11 wherein:

each of the flow branches includes a third zone; and, an exit orifice common to each of the third zones.

16. The apparatus of claim 9 wherein .alpha. is between 40° and 120°.

17. The apparatus of claim 9 wherein the upper wall and the lower wall of the flow channel in the second zone are disposed at an angle .beta. to one another to affect a gradual decrease in a channel height in the second zone.

18. The apparatus of claim 17 wherein angle .beta. is between 5° and 30°.

19. A flow directing insert for use in an apparatus for directing the flow of polymeric material from a supply source to one or more shaping dies, the apparatus having a flow channel therein, the flow channel having an inlet communicating with a supply orifice for receiving the material, the flow channel being defined by opposite side walls, an upper wall, and a lower wall, the flow channel including a downstream region having a first zone in flow communication with the inlet, and a second zone in flow communication with the first zone, the first and second zones being associated with a common centerline, the side walls of the second zone diverging in opposite lateral directions generally symmetrically about the centerline and associated with an angle, .alpha., the upper wall being in a common plane throughout the first and second zones, the flow directing insert for location within the second zone for positioning adjacent to the lower wall and spaced from the opposite side walls, the flow directing insert having a triangular bottom surface, a triangular top surface non-parallel to the bottom surface, and sloping side surfaces extending between the bottom surface and the top surface, the side surfaces intersecting to form a forward edge.